1. Field of the Invention
The present invention relates to golf balls. Specifically, the present invention relates to golf balls including a solid core and a cover.
2. Description of the Related Art
Golf players' foremost requirement for golf balls is flight performance. Golf players place importance on flight performance upon shots with a driver, a long iron, and a middle iron. Flight performance correlates with the resilience performance of a golf ball. When a golf ball with excellent resilience performance is hit, the golf ball flies at a high speed, thereby achieving a large flight distance.
An appropriate trajectory height is required in order to achieve a large flight distance. A trajectory height depends on a spin rate and a launch angle. In a golf ball which achieves a high trajectory by a high spin rate, a flight distance is insufficient. In a golf ball which achieves a high trajectory by a high launch angle, a large flight distance is obtained. By using a core having an outer-hard/inner-soft structure, a low spin rate and a high launch angle can be achieved.
JPH2-264674 (U.S. Pat. No. 5,072,944) discloses a golf ball with a core consisting of a center and an outer layer. The center is flexible, and the outer layer is hard. The core suppresses a spin rate.
However, in the golf ball disclosed in JPH2-264674, the structure of the core is complicated. The core produces an energy loss when being hit. In addition, the core has inferior durability.
JPH6-98949 (U.S. Pat. No. 5,516,110) discloses a golf ball having a constant hardness between: a point which is located at a distant of 5 mm from a central point; and a point which is located at a distant of 10 mm from the central point. A similar golf ball is also disclosed in JPH6-154357 (U.S. Pat. No. 5,403,010).
However, in the golf ball disclosed in JPH6-98949, a range where the hardness is constant is narrow. The golf ball has inferior resilience performance. Similarly, the golf ball disclosed in JPH6-154357 also has inferior resilience performance.
JPH7-112036 (U.S. Pat. No. 5,562,287) discloses a golf ball having a small difference between a central hardness and a surface hardness of a core. The core contributes to the resilience performance of the golf ball.
However, in the golf ball disclosed in JPH7-112036, a spin rate is excessive. The golf ball has a small flight distance.
JP2002-764 (US2002/032077) discloses a golf ball having a great difference between a central hardness and a surface hardness of a core. A similar golf ball is also disclosed in JP2002-765 (US2002/019269).
However, the golfball disclosed in JP2002-764 has inferior resilience performance. Similarly, the golf ball disclosed in JP2002-765 also has inferior resilience performance.
JP2003-33447 (US2003/032501) discloses a golf ball with a core for which a rubber composition includes a polysulfide. The polysulfide contributes to the resilience performance of the golf ball.
However, in the golf ball disclosed in JP2003-33447, a spin rate is excessive. The golf ball has inferior flight performance.
JP2008-194473 (US2008/194357 and US2008/312008) discloses a golf ball having a great difference between a central hardness and a surface hardness of a core. A similar golf ball is also disclosed in JP2010-22504.
However, in the golfball disclosed in JP2008-194473, there is a zone in which a hardness decreases from the central point of the core toward the surface of the core. The golf ball has inferior resilience performance. In the golf ball, a spin rate is excessive. The golf ball has inferior flight performance. Similarly, the golf ball disclosed in JP2010-22504 also has inferior flight performance.
An object of the present invention is to provide a golf ball having excellent flight performance.